Reflective power monitoring for microwave applications

ABSTRACT

A system and method for supplying microwave energy to tissue for microwave therapy includes an electrosurgical generator having an output for coupling to a surgical instrument. The electrosurgical generator includes a microwave energy source and a controller for controlling the operation of the electrosurgical generator. The surgical instrument, coupled to the electrosurgical generator, includes a microwave antenna for delivering microwave energy from the microwave energy source. The controller of the electrosurgical generator is operable for causing the electrosurgical generator to apply at least two pulses of microwave energy.

BACKGROUND

1. Technical Field

The present disclosure relates generally to medical/surgical ablationprocedures. More particularly, the present disclosure relates to devicesand microwave radiation delivery procedures utilizing microwave antennaassemblies and methods of controlling the delivery of microwaveradiation to tissue.

2. Background of Related Art

In the treatment of diseases such as cancer, certain types of cancercells have been found to denature at elevated temperatures (which areslightly lower than temperatures normally injurious to healthy cells).These types of treatments, known generally as hyperthermia therapy,typically utilize electromagnetic radiation to heat diseased cells totemperatures above 41° C. while maintaining adjacent healthy cells atlower temperatures where irreversible cell destruction will not occur.Other procedures utilizing electromagnetic radiation to heat tissue alsoinclude ablation and coagulation of the tissue. Such microwave ablationprocedures, e.g., such as those performed for menorrhagia, are typicallydone to ablate and coagulate the targeted tissue to denature or kill it.Additionally, microwave therapy may be used in the treatment of tissueand organs such as the prostate, heart, and liver.

One advantage of microwave therapy is that microwave energy is able tonon-invasively penetrate the skin to reach underlying tissue. Unlike lowfrequency RF therapy, which heats tissue with current, microwave therapyheats tissue within the electromagnetic field generated by a microwaveantenna. The electromagnetic field generated by the microwave antennagenerates a predictably large and/or uniform ablation region.

A second advantage of microwave therapy is that energy is rapidlydelivered to the target tissue resulting in the reduction of surgicalprocedure time. During a typical surgical procedure microwave energyrapidly heats tissue to a target temperature and maintains the tissueabove the target temperature for a required period of time.

Rapid delivery of heat to tissue may also result in the unwanted heatingof healthy tissue or overheating of the target tissue. Unwanted heatingof healthy tissue may be the result of creating an electromagnetic fieldlarger than is required or excessive energy delivery. Overheating oftissue may result from excessive energy delivery or inconsistent heatingof the target tissue.

Thus, the non-invasive use of microwave energy requires a great deal ofcontrol. This is partly why a more direct and precise method ofdelivering microwave radiation has been sought. The present disclosureprovides a system for supplying microwave and various methods ofdelivering microwave radiation to tissue.

SUMMARY

The present disclosure relates generally medical/surgical ablationprocedures. More particularly, the present disclosure relates tomonitoring characteristics of microwave radiation delivery proceduresutilizing microwave antenna assemblies configured for direct insertioninto tissue and methods of controlling the delivery of microwaveradiation to tissue.

A system for supplying microwave energy for microwave therapy of thepresent disclosure comprises an electrosurgical generator including amicrowave energy source and a controller for controlling the operationof an electrosurgical generator. The electrosurgical generator includesan output for coupling to a surgical instrument which includes amicrowave antenna for delivering microwave energy. The controller beingoperable for causing the electrosurgical generator to apply two or morepulses of microwave energy to the tissue.

The controller may be configured to measure an electrical characteristicof at least one of the at least two pulses of microwave energy.Controller may be responsive to the measured electrical characteristicof at least one of the at least two pulses of microwave energy fordetermining at least one pulse parameter of a subsequent microwaveenergy pulse. The pulse parameter may be selected from a groupconsisting of power, frequency, pulse duration, pulse shape, pulse dutycycle and time between pulses. The electrical characteristic may berelated to reflective power. The controller may vary the power of eachof the pulses of microwave energy. Controller may also be responsive toa control input from an operator for modifying any one of the at leastone pulse parameters.

A method for applying microwave energy to target tissues includes thesteps of positioning the microwave energy delivery device into oradjacent a portion of the target tissue and delivering at least twopulses of microwave energy to the target tissue wherein a substantialportion of the microwave energy is reduced between the at least twopulses of microwave energy. The delivering step may include the steps ofheating a portion of the target tissue to a target temperature andmaintaining the portion of the target tissue at or above the targettemperature.

In another embodiment the step of delivering the at least two pulses ofmicrowave energy may include the step of selecting and varying at leastone parameter thereof. The step of selecting and varying the at leastone parameter may further include the step of selecting the at least oneparameter from a group consisting of power, frequency, pulse duration,pulse shape, phase, pulse duty cycle and time between pulses.

In yet another embodiment the step of delivering the at least two pulsesof microwave energy may include the step of varying at least one of theat least two pulses of microwave energy in accordance with at least onecharacteristic of an electrical transient of one of the at least twopulses of microwave energy.

In yet another embodiment the step of delivering the at least two pulsesof microwave energy may include the step of selecting parameters of theat least two pulses of microwave energy such that a rise in targettissue temperature is about zero.

In yet another embodiment the step of delivering the at least two pulsesof microwave energy may include the steps of measuring at least onecharacteristic of the target tissue in response to one of the at leasttwo pulses of microwave energy and in accordance with the at least onecharacteristic of the target tissue, determining whether to change themicrowave energy parameters. The step of measuring the at least onecharacteristic of tissue may further include the step of selecting theat least one characteristic of the tissue from a group consisting of acharacteristic related to reflective power, a characteristic related totissue temperature, and a characteristic related to tissue impedance.The method may further include the step of determining a response of thetarget tissue to one of the at least two pulses of microwave energy.

In yet another embodiment the step of delivering the at least two pulsesof microwave energy may include the steps of measuring at least onecharacteristic of one of the at least two pulses of microwave energy andin accordance with the at least one measured characteristic, determiningwhether to change at least one microwave energy parameter. The step ofmeasuring the at least one characteristic of one of the at least twopulses of microwave energy may include the step of selecting at leastone characteristic related to reflective power. The method may furtherinclude the steps of measuring at least one characteristic of one of theat least two pulses of microwave energy and in accordance with the atleast one measured characteristic, at least one of determining whetherto terminate the application of microwave energy to tissue, and usingthe at least one measured characteristic to determine a set of microwaveenergy parameters for a subsequent pulse of microwave energy. The atleast one microwave energy parameter may be selected from a groupconsisting of power, frequency, pulse duration, pulse shape, phase,pulse duty cycle and time between pulses.

In yet another embodiment the method of positioning the microwave energydelivery device into or adjacent a portion of the target tissue anddelivering at least two pulses of microwave energy may include the stepsof measuring at least one characteristic of the target tissue inresponse to one of the at least two pulses of microwave energy; and inaccordance with the at least one characteristic of the target tissue, atleast one of determining whether to terminate the application ofmicrowave energy, and using the at least one characteristic of thetarget tissue to determine a set of microwave energy parameters forapplying a subsequent pulse of microwave energy. The set of microwaveenergy parameters for the application of a subsequent pulse of microwaveenergy may includes a magnitude of a microwave power, a pulse shape,pulse duration and any combination thereof.

The delivery of microwave energy may be terminated upon a determinationthat a predetermined condition is satisfied. The predetermined conditionis selected from a group consisting of treatment duration, a conditionrelated to temperature and a condition related to reflective power.

In yet another embodiment of the present disclosure a method forapplying microwave energy to a target tissue, includes the steps ofpositioning a microwave energy delivery device into or adjacent aportion of the target tissue, delivering at least two pulses ofmicrowave energy to the target tissue wherein a substantial portion ofthe microwave energy is reduced between the at least two pulses ofmicrowave energy, measuring at least one of the temperature of atransmission line and a temperature of the microwave energy deliverydevice, and varying at least one of the at least two pulses of microwaveenergy in response to the at least one of the measured temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a system for supplying microwaveenergy for microwave therapy, according to an embodiment of the presentdisclosure;

FIG. 2 illustrates various waveforms associated with electricalcharacteristics of microwave therapy performed with the system forsupplying microwave energy of FIG. 1;

FIG. 3 illustrates microwave pulses generated by the system forsupplying microwave energy of FIG. 1 according to a method of thepresent disclosure; and

FIG. 4 illustrates microwave pulses generated by the system forsupplying microwave energy of FIG. 1 according to another method of thepresent disclosure.

DETAILED DESCRIPTION

Embodiments of the presently disclosed microwave antenna assembly willnow be described in detail with reference to the drawing figures whereinlike reference numerals identify similar or identical elements. As usedherein and as is traditional, the term “distal” refers to the portion ofthe device or instrument that is furthest from the user and the term“proximal” refers to the portion of the device or instrument that isclosest to the user. In addition, terms such as “above”, “below”,“forward”, “rearward”, etc. refer to the orientation of the figures orthe direction of components and are simply used for convenience ofdescription.

Two factors in the treatment of diseased tissue with microwave therapyinclude the proper positioning and placement of a microwave antennarelative to the target tissue, and the delivery of microwave energy.

The step of positioning and placement of a microwave antenna, relativeto target tissue, is dependant upon the location of the target tissue.One method of accessing target tissue involves the insertion of themicrowave antenna into an existing body lumen, such as, for example,accessing the inner chambers of the heart through the vascular system,accessing the prostate by inserting a microwave antenna through theurethra or accessing the lungs and esophagus by inserting a microwaveantenna through the mouth cavity. Various methods and devices areprovided for in U.S. Pat. No. 5,916,241, entitled “Device and Method forAsymmetrical Thermal Therapy with Helical Dipole Microwave Antenna”, toRudie et al., describing accessing the prostate through the urethra, andU.S. Pat. No. 7,070,595, entitled “Radio-Frequency Based Catheter Systemand Method for Ablating Biological Tissue”, to Ormsby et al., describingaccessing cardiac tissue through the vascular system.

Another method of accessing target tissue involves direct insertion of adevice into tissue. A microwave antenna may be formed or insertedthrough a delivery device such as, for example, an introducer orcatheter, as described in U.S. Pat. No. 6,355,033, entitled “TrackAblation Devices and Methods”, to Moorman et al. The microwave antennamay also be sufficiently strong to allow direct insertion into tissue asdescribed in U.S. Pat. No. 6,878,147, entitled “High Strength MicrowaveAntenna Assemblies”, to Prakash et al. Yet another microwave device maybe inserted into tissue in a first condition followed by deployment ofthe distal penetrating portion thereof to a second condition therebyforming a microwave antenna at the distal end of the assembly asdescribed in U.S. application Ser. No. 10/272,314, entitled “MicrowaveAntenna Having a Curved Configuration”, to Prakash et al.

The delivery of microwave energy is also dependant upon the location ofthe target tissue. Ormsby et al. '595 describes delivery of RF energy inthe vascular system that generates and delivers a continuous train ofradio frequency energy pulses at an output frequency for transmission ina transmission line to a shapeable RF antenna. The output frequency isadjusted based on the sensed reflective signal in order to substantiallymatch the transmission line impedance with the shapeable RF antenna andthe biological tissue load impedance. Rudie et al. '241 describesdelivery of a continuous train of microwave energy in the urologicalsystem. In both Ormsby et al. '595 and Rudie et al. '241 the outputfrequency is adjusted to reduce the risk of overheating by theelectrical transmission and to prevent damage to the lumen in which thetransmission line is contained.

The present disclosure provides a system for supplying microwave energyand various methods of delivering microwave radiation to tissue with amicrowave device. A microwave device of the system for supplyingmicrowave energy may be inserted directly into tissue, inserted througha lumen, such as, for example, a vein, needle or catheter, placed intothe body during surgery by a clinician or positioned in the body byother suitable methods or means known in the art. While the methoddescribed hereinbelow is targeted toward microwave ablation and thecomplete destruction of target tissue, the methods may be used withother microwave therapies in which the target tissue is partiallydamaged, e.g. in order to prevent the electrical conduction in hearttissue.

Referring now to FIG. 1, a system for supplying microwave energy formicrowave therapy, according to an embodiment of the present disclosure,is shown as 10. The system for supplying microwave energy 10 includes anelectrosurgical generator 20, with a controller 22 for controlling theoperation of the electrosurgical generator 20, an electrosurgicalinstrument or device 30 coupled to an output 24 on the electrosurgicalgenerator 20. Device 30 includes a microwave antenna 32, for delivery ofmicrowave energy, coupled to a transmission line 34, which electricallyconnects antenna 32 to output 24 on electrosurgical generator 20.Electrosurgical generator 20 includes an operator interface 40 having akeypad 42 and a display 44 for entering microwave energy parameters byan operator.

Device 30 is illustrated as a microwave antenna sufficiently strong toallow direct insertion or penetration into tissue, although microwaveantenna may be any device suitably capable of delivering microwaveenergy or the like.

System 10 may include one or more sensors to measure various parametersassociated with the delivery of microwave energy. For example, device 30and/or transmission line 34 may include one or more suitable temperaturemeasuring sensor 35, 36. As seen in FIG. 1, a first sensor 35 isconnected to transmission line 34 and a second sensor 36 is connected toantenna 32. Sensors 35, 36 may be incorporated into the construction of,or formed as part of, the transmission line 34 and/or antenna 32 and mayconnect to electrosurgical generator 20 through transmission line 34 andconnector 24. Alternatively, sensors 35, 36 may be placed on, in or neartransmission line 34 or antenna 32, such as, for example, apercutaneously insertable temperature or impedance sensor or a surfacemountable temperature sensor and may connect directly to electrosurgicalgenerator 20.

FIG. 2 illustrates various waveforms associated with electricalcharacteristics of microwave therapy performed with the system 10 ofFIG. 1. A first waveform is forward power “FP” or power supplied to theantenna 32 by the electrosurgical generator 20. A second waveform istissue temperature “T” measured by a temperature measuring sensor 36. Athird waveform is reflected power “RP” or power that is not transmittedby the antenna 32 to tissue. Each waveform is related to the delivery ofmicrowave energy.

During operation of system 10, as seen in FIG. 2, during an initial 40seconds of operation, the temperature “T” of tissue begins to rise asenergy is delivered to the tissue. Between 40 and 50 seconds ofoperation, the tissue temperature “T” exceeds 100° C. and the water orliquid in the patient tissue “PT” (see FIG. 1) begins to boil. Between40 and 90 seconds of operation, tissue temperature “T” is maintained ata temperature slightly over 100° C. Between 90 and 100 seconds ofoperation, the water or liquid in the patient tissue “PT” is boiled offand the tissue temperature “T” again begins to rise. The energy suppliedto patient tissue “PT”, from system 10, begins to char the tissue “PT”and potentially irreversible changes or effects result in the tissue“PT”.

FIG. 2 is provided as an example of waveforms associated with electricalcharacteristics of microwave therapy and should not be construed aslimiting hereto. The systems and methods described herein may utilizeother suitable waveforms associated with microwave therapy. The shape ofthe waveforms are dependent upon many factors (e.g., the type of tissuereceiving the energy, the desired therapeutic result, the size and typeof microwave antenna used and/or the power and frequency of themicrowave energy.) For example, tissue with high fluid content, such as,for example, kidney tissue, liver tissue or cardiovascular tissue, mayrequire more energy and time to heat than tissue with low fluid content,such as, for example, lung tissue.

The desired therapeutic result may also impact the shape of theindividual waveforms. For example, during tissue ablation tissuetemperature may rise to about 60° C. to 70° C. and the clinician maymaintain the tissue at a temperature for a target amount of time. Othertreatments may require raising tissue to a temperature less than 60° C.or may not require maintaining the tissue at a target temperature.

With continued reference to FIG. 2, electrosurgical generator 20maintains forward power “FP” at approximately 60 Watts for the durationof the delivery of microwave energy.

During the initial delivery of microwave energy to patient tissue “PT”,between 0 and 70 seconds, the antenna 32 is matched to the tissueproperties and reflective power “RP” is at a minimum at a level slightlymore than zero. Between 90 and 100 seconds the properties of patienttissue “PT” begin to change and reflective power “RP” begins toincrease. This increase in reflective power “RP” indicates that theantenna 32 is less “tuned”, or matched to the properties of tissue “PT”,and less power is delivered to tissue “PT”. At this point,electrosurgical generator 20 continues to maintain constant forwardpower “FP” and the antenna 32 become less efficient and reflective powercontinues to increase.

Reflective power may be reduced by adjusting the frequency of thedelivered microwave energy to match the impedance of the damaged tissue.The present disclosure provides for a system and method of controllingthe delivery of microwave energy in order to prevent and/or minimize achange in tissue impedance, thus preventing further increases inreflective power.

With reference to FIGS. 1 and 2, forward power “FP”, temperature “T” andreflective power “RP” are waveforms, or electrical transients, relatedto microwave energy delivery. Controller 22 may be responsive to atleast one characteristic of an electrical transient, or characteristicthat occurs during at least one individual pulse of microwave energy.The characteristic of an electrical transient, or property of a givenwaveform, may include the rate of change of reflective power “ΔRP”, rateof change of temperature “ΔT”, rate of change of impedance (not shown),rate of change of forward power “ΔFP” or a value from the waveform. Thetransient may also be related to a particular phenomenon that occurs inthe waveform, such as, for example, the heating time “HT” of tissue inresponse to the delivery of microwave power or the reaction time oftissue temperature in response to the removal of the delivery and/ortransmission of microwave energy.

Controller 22 may also be responsive to an individual measurementrelated to microwave energy delivery, such as, for example, aninstantaneous or time weighted measurement of temperature, power orimpedance.

With reference to FIGS. 1 and 3, controller 22 is operable to cause theelectrosurgical generator 20 to apply two or more pulses 100-100 n ofmicrowave energy to tissue. Each pulse of microwave energy is defined byvarious parameters such as, for example, frequency of the microwaveenergy, amount of power delivered by the microwave energy, the timeduration of the microwave energy pulse, and the shape of the microwaveenergy pulse. For example, the pulse may be a square wave pulse 100, asillustrated in FIG. 1, a pulse with a magnitude or a pulse having ashape that may be dependant of the amount of energy delivered to tissue.

Controller 22 is operable to remove, reduce or turn off the delivery ofenergy 110-110 n to patient tissue “PT” between energy pulses 100-100 n.With reference to FIG. 1, removal of energy delivery to patient tissue“PT” may be accomplished by switching the delivery of energy from theoutput 24 to a secondary load 26. Energy may be reduced by automaticallyadjusting, or by substantially reducing, the power delivered to theoutput 24. Alternatively, controller may turn off the energy deliveredto the output 24.

The energy pulse generated by the controller, irrespective of the methodof generation, is characterized by delivery of energy to tissue for anamount of time followed by a second amount of time wherein the energydelivered to tissue is substantially reduced. Preferably, the energydelivered to tissue during the second amount of time is reduced to alevel that is metabolically insignificant, or equal to the basalmetabolism level of about 1 W/kg of tissue.

The controller 22 may include means for switching (not explicitly shown)the delivery of energy to secondary load 26. The means for switching maybe a separate device included in the electrosurgical generator 20 or itmay be an external device controlled by the electrosurgical generator20. Secondary load may be a resistive load, for absorbing and/ordissipating the energy, an additional electrosurgical device, such as,for example, a microwave antenna, or any combination thereof.

With continued reference to FIG. 1, controller 22 measures an electricalcharacteristic of the microwave energy delivered to patient tissue “PT”and is capable of adjusting one or more pulse parameters based on themeasured electrical characteristic. Measurements may be made orperformed by the electrosurgical generator 20, the controller 22, asensor 36 in the device 30, a sensor 35 in transmission cable 34 or by aseparate sensor (not explicitly shown) coupled to the electrosurgicalgenerator 20. Controller 22 may vary the pulse parameter of the currentmicrowave energy pulse or may vary the pulse parameter of any subsequentenergy pulse. Controller 22 may also utilize the measured electricalcharacteristics of microwave energy to determine if the energy deliveryshould continue or if energy delivery should be terminated.

FIG. 3 illustrates at least one embodiment of microwave energy deliveredto patient tissue “PT” by the system 10 of the present disclosure. Themicrowave energy is defined by a series of energy pulses 100 to 100 nseparated by delays 110-110 n during which delays 110-110 n the energydelivered to tissue “PT” is removed or is substantially reduced. Firstmicrowave pulse 100 is followed by a first delay 110, followed by one ormore subsequent energy pulses 100 a-n each of which is separated by adelay 110 a-110 n−1.

Providing microwave energy as a series of pulses, separated by delays,wherein the energy is removed or substantially reduced, improvesdelivery and dispersion of heat within the target tissue. Removing powerprovides a relaxation period for the tissue “PT” and enables the tissue“PT” to re-hydrate and recover. Microwave energy is then reapplied totissue “PT” after the delay or relaxation period and the process may berepeated as needed and/or desired. This periodic redistribution of heatthrough the tissue “PT” during the relaxation period, followed by there-delivery and/or re-transmission of microwave energy, improves theablation size and results in a more predictable ablation area.

Measurements may be continuously or periodically performed during energydelivery, the relaxation period or any combination thereof.

During the delivery of microwave energy, antenna mismatch or reflectedpower may produce standing waves in the transmission cable 34 and/ordevice 30 resulting in an increase in the voltage standing wave ratio(hereinafter “VSWR”). Standing waves produce hot spots in thetransmission cable 34 at various locations along a length thereof, thehot spots are typically spaced every ½ wavelength along the cable. Thelocation and/or the temperature of the hot spots correspond to aspecific characteristic of the standing wave or to the value of theVSWR. Continuous delivery of microwave energy may result in these hotspots exceeding a maximum expectable temperature and may result inpatient or clinician injury or damage of equipment.

Providing a series of microwave energy pulses separated by delays, asdisclosed herein, may result in even distribution of heat along thetransmission path. For example, the delay between consecutive pulses mayallow the thermal energy to conduct away from each hot spot resulting ina more even distribution of temperature. In addition, pulsing ofmicrowave energy may change at least one characteristic of the standingwave. For example, the standing wave may change phase or relaxation ofpatient tissue may change the VSWR. This may result in repositioning orshifting the hot spots between a pulse of microwave energy and asubsequent pulse. Shifting of the location of the hot spots betweenpulses may result in a more even distribution of heating and/ortemperature along the transmission cable 34.

Microwave energy therapy parameters are used to determine the number ofmicrowave energy pulses, the duration and power of each individualenergy pulse, the delay between energy pulses and the total duration ofthe procedure. Individual pulse parameters include the parametersassociated with each individual pulse, such as, for example, theduration of the pulse, the frequency of the pulse, the power setting ofthe pulse and the delay after each individual pulse. With reference toFIG. 1, parameters may be predetermined by a clinician and enteredthrough the operator interface 40, may be determined by the controller22 or may be entered through the keypad 42 by the clinician and adjustedby controller 22. Pulse parameters may be adjusted for each individualpulse. Alternatively, some parameters, such as the frequency of themicrowave energy, may be consistent throughout the duration of themicrowave energy therapy.

Controller 22 may be responsive to the measured electricalcharacteristics of one or more microwave energy pulses. The measuredelectrical characteristic may be related to reflective power, such as,for example, reflective power magnitude, reflective power phase,reflection loss, reflection coefficient, voltage standing wave ratio(VSWR), return loss, or mismatch loss VSWR. The measured electricalcharacteristic may be a sensed parameter such as, for example,temperature of the transmission line 34 measured by sensor 35 or thetemperature of the device 30 measured by device sensor 36. In operation,controller 22 may adjust one or more parameters of the current energypulse or of a subsequent microwave energy pulse.

In the operation of one embodiment of the present disclosure, aclinician may view the various electrical characteristics related toreflective power, via the user interface 40 and display 44. Theclinician may make adjustments to the microwave energy therapyparameters or pulse parameters based on the various electricalcharacteristics. The clinician may adjust one or more parameters of thecurrent pulse or of a subsequent pulse or may adjust one or moremicrowave energy therapy parameters.

A first method of applying microwave therapy to tissue includes thesteps of applying a first pulse of microwave energy to tissue, removinga substantial portion of the microwave energy from the tissue andapplying at least one subsequent microwave energy pulse to tissue.Removing a substantial portion of the microwave energy may include:reducing the energy to a level that allows the tissue relaxes, reducingthe energy to a level that maintains tissue temperature, reducing energyto a level that is metabolically insignificant, or reducing thedelivered energy to zero.

As discussed hereinabove and with reference to FIG. 1, controller 22 maybe responsive to at least one characteristic of an electrical transientthat occurs during at least one individual pulse of microwave energy ormay be responsive to a measurement related to microwave energy delivery.The step of applying at least one subsequent microwave energy pulse totissue may include the step of selecting and varying at least oneparameter for the subsequent microwave energy pulse. The parameter maybe varied in accordance to an individual measurement or may be varied inaccordance with at least one characteristic of an electrical transientthat occurs during an individual pulse of microwave energy. The variedparameter may include, and is not limited to forward power, frequency,energy, pulse duration, pulse shape and termination of energyapplication.

FIG. 4 provides an example of energy delivery in accordance with oneembodiment of the present disclosure. With reference to FIGS. 1 and 4,initial energy pulse 200 is applied with a forward power setting of 80watts. For the initial energy pulse 200, controller 22 may be responsiveto a characteristic of tissue “PT”, a characteristic of the microwaveenergy pulse and/or a characteristic of an electrical transient.Controller 22 terminates the initial energy pulse 200 when a rate ofchange of tissue temperature “T” exceeds a threshold value. Thethreshold value of the rate of change of tissue temperature may be apredetermined value, an operator entered value and/or a value calculatedby controller 22. Alternatively, duration of the initial energy pulse200 may be shortened or a pulse parameter, such as power, may bemodified. In FIG. 4, the rate of change of temperature “T” terminatesthe initial energy pulse 200 at time “T1”.

After the termination of the initial energy pulse 200, the temperature“T” may continue to rise for a period to time “T2” due to the heatdispersing within the target tissue. The duration of a first off period210 having a period of time “T3” that may be equal to a predeterminedamount of time. Controller 22 may vary the duration of the first offperiod 210 in response to a characteristic of tissue, a characteristicof a previous microwave energy pulse and/or a characteristic of anelectrical transient. In FIG. 4, the first off period 210 terminates attime “T4” and the first subsequent energy pulse 200 a is applied to thetarget tissue.

The individual pulse parameters of the first subsequent energy pulse 200a may be varied according to a characteristic of tissue, acharacteristic of the microwave energy pulse or a characteristic of anelectrical transient, such as, for example, the duration of the firstoff period 210. A long “off” period may indicate that the target tissuecontains excessive heat energy and the individual pulse parameter may beadjusted to deliver less energy. Alternatively, a short “off” period mayindicate that the target tissue quickly disperses the heat energy in thetarget tissue and the energy of the first subsequent energy pulse shouldremain the same or may be increased.

Control of the first subsequent energy pulse 200 a may be similar to thecontrol of the initial energy pulse 200 or control may be responsive toa different measured parameter or different characteristic of anelectrical transient. As the temperature “T” of tissue “PT” increases,steam begins to form at the molecular level and/or the tissue begins totransform. Steam generation and phase transformation effect thermaldelivery and dispersion within the target volume and reflective power“RP” will begin to increase. At this point, parameters of the subsequentenergy pulses 200 c-2001 are selected as to not appreciably heat thetissue “PT”.

With continued reference to FIG. 4, reflective power “RP” may remainrelatively constant during the initial delivery of electrosurgicalenergy, e.g. between 0 and about 45 seconds. As the temperature of thetissue increase, e.g. between about 45 seconds and about 60 seconds, thereflective power “RP” increases. When the reflective power “RP” exceedsa threshold value, the energy delivery may be modified, for example, thepower level may be decreased and/or the duration between energysubsequent energy pulses may be increased. The threshold value forreflective power “RP” may vary between about 1% and about 20% ofreflective energy, typically about 5% of reflective energy.

A further embodiment of the present method includes the step ofmeasuring at least one characteristic of the microwave energy pulse anddetermining whether to change the microwave energy parameters. Thecharacteristic may be related to reflective power “RP”, such as, forexample, the instantaneous measurement or the rate of change ofreflective power “ΔRP”. Energy pulse parameters are selected as tocontrol or maintain selected characteristics.

Reflective power “RP” and the rate of change of reflective power “ΔRP”can be used as an indicator of the condition of tissue. Formation ofsteam, while conducive to thermal delivery of heat, removes moisturefrom tissue and changes the impedance of the tissue. A change of tissueimpedance creates an imbalance between the antenna and the tissue “PT”which causes an increase in reflective power “RP”. In selectingparameters to maintain or reduce reflective power “RP”, the system 10,while creating some steam, does not damage tissue “PT” resulting in morepredictable and/or larger ablation sizes.

According to a further method of the present disclosure, at least onecharacteristic of the tissue “PT” is measured in response to the appliedfirst pulse. The controller 22, in accordance with the measuredcharacteristic may terminate the delivery of microwave energy to tissue.The controller 22 may also use the measured characteristic to determinea set of microwave energy parameters for a subsequent microwave energypulse. The steps are then repeated for the subsequent pulses.

In a further embodiment of the present method, the set of microwaveenergy parameters for the subsequent microwave energy pulse may includeat least one of a magnitude of a starting microwave power, pulse shape,pulse duration and any combination thereof.

In a further method of the present disclosure, the delivery ofelectrosurgical energy, and/or the “off” period, may be adjusted fortemperature management of portions of the electrosurgical system ortemperature management of patient tissue “PT”. With reference to FIG. 1,controller 22 of the electrosurgical generator 20 monitors at least oneof the temperature of the transmission line 34 via line sensor 35 and/orthe temperature of the device via device sensor 36. Device 20 mayinclude one or more tissue temperature measuring sensors (not shown) ortissue measurement sensors may connect directly to electrosurgicalgenerator. With reference to FIG. 4, energy delivery may beintermittently reduced or paused, or the “off” period may be increased,to allow the transmission line 34, device 20 or patient tissue “PT” tocool.

The system for supplying microwave energy for microwave therapy and themethods of use discussed above are not limited to microwave antennasused for hyperthermic, ablation, and coagulation treatments but mayinclude any number of further microwave antenna applications. Whileseveral embodiments of the disclosure have been shown in the drawingsand/or discussed herein, it is not intended that the disclosure belimited thereto, as it is intended that the disclosure be as broad inscope as the art will allow and that the specification be read likewise.Modification of the above-described system and methods for using thesame, and variations of aspects of the disclosure that are obvious tothose of skill in the art are intended to be within the scope of theclaims. Therefore, the above description should not be construed aslimiting, but merely as exemplifications of particular embodiments.

1. A method for applying microwave energy to a target tissue, comprisingthe steps of: positioning a microwave energy delivery device one of intoa portion of target tissue and adjacent a portion of target tissue;delivering at least two pulses of microwave energy to the target tissuewherein a substantial portion of the microwave energy is reduced betweenthe at least two pulses of microwave energy;
 2. The method as in claim1, wherein the step of delivering the at least two pulses of microwaveenergy further include the steps of: heating a portion of the targettissue to a target temperature; and maintaining the portion of thetarget tissue at or above the target temperature.
 3. A method as inclaim 1, wherein the step of delivering the at least two pulses ofmicrowave energy includes the step of selecting and varying at least oneparameter thereof.
 4. A method as in claim 1, wherein the step ofdelivering the at least two pulses of microwave energy includes the stepof varying at least one of the at least two pulses of microwave energyin accordance with at least one characteristic of an electricaltransient of one of the at least two pulses of microwave energy.
 5. Amethod as in claim 3, wherein the step of selecting and varying the atleast one parameter includes the step of selecting the at least oneparameter from a group consisting of power, frequency, pulse duration,pulse shape, phase, pulse duty cycle and time between pulses.
 6. Amethod as in claim 1, wherein the step of delivering the at least twopulses of microwave energy includes the step of selecting parameters ofthe at least two pulses of microwave energy such that a rise in targettissue temperature is about zero.
 7. A method as in claim 1, wherein thestep of delivering the at least two pulses of microwave energy includethe steps of: measuring at least one characteristic of the target tissuein response to one of the at least two pulses of microwave energy; andin accordance with the at least one characteristic of the target tissue,determining whether to change the microwave energy parameters.
 8. Amethod as in claim 7, wherein the step of measuring the at least onecharacteristic of target tissue includes the step of selecting the atleast one characteristic of the target tissue from a group consisting ofa characteristic related to reflective power, a characteristic relatedto tissue temperature, and a characteristic related to tissue impedance.9. A method as in claim 7, further comprising the step of determining aresponse of the target tissue to one of the at least two pulses ofmicrowave energy.
 10. A method as in claim 1, further comprising thesteps of: measuring at least one characteristic of the target tissue inresponse to one of the at least two pulses of microwave energy; and inaccordance with the at least one characteristic of the target tissue, atleast one of determining whether to terminate the application ofmicrowave energy, and using the at least one characteristic of thetarget tissue to determine a set of microwave energy parameters forapplying a subsequent pulse of microwave energy.
 11. A method as inclaim 10, wherein the set of microwave energy parameters for theapplication of a subsequent pulse of microwave energy includes amagnitude of a microwave power, a pulse shape, and a pulse duration. 12.A method as in claim 1, wherein the step of delivering the at least twopulses of microwave energy comprises the steps of: measuring at leastone characteristic of one of the at least two pulses of microwaveenergy; and in accordance with the at least one measured characteristic,determining whether to change at least one microwave energy parameter.13. A method as in claim 12, wherein the step of measuring the at leastone characteristic of one of the at least two pulses of microwave energyincludes the step of selecting at least one characteristic related toreflective power.
 14. A method as in claim 12, further comprising thesteps of: measuring at least one characteristic of one of the at leasttwo pulses of microwave energy; and in accordance with the at least onemeasured characteristic, at least one of determining whether toterminate the application of microwave energy to tissue, and using theat least one measured characteristic to determine a set of microwaveenergy parameters for a subsequent pulse of microwave energy.
 15. Amethod as in claim 12, wherein the at least one microwave energyparameter is selected from a group consisting of power, frequency, pulseduration, pulse shape, phase, pulse duty cycle and time between pulses.16. A method as in claim 1, further comprising the step of terminatingthe delivery of microwave energy upon a determination that apredetermined condition is satisfied.
 17. A method as in claim 16,wherein the predetermined condition is selected from a group consistingof treatment duration, a condition related to temperature, a conditionrelated to energy duty cycle and a condition related to reflectivepower.
 18. A system for supplying microwave energy to tissue,comprising: an electrosurgical generator including a microwave energysource and a controller for controlling the operation of theelectrosurgical generator, the electrosurgical generator having anoutput for coupling to an instrument including a microwave antenna fordelivering microwave energy, the controller operable to cause theelectrosurgical generator to apply at least two pulses of microwaveenergy.
 19. A system as in claim 18, wherein the controller isconfigured to measure an electrical characteristic of at least one ofthe at least two pulses of microwave energy.
 20. A system as in claim19, wherein the controller is responsive to the measured electricalcharacteristic of at least one of the at least two pulses of microwaveenergy for determining at least one pulse parameter of a subsequentmicrowave energy pulse.
 21. A system as in claim 20, wherein the atleast one pulse parameter is selected from a group consisting of power,frequency, pulse duration, pulse shape, phase, pulse duty cycle and timebetween pulses.
 22. A system as in claim 20, wherein the electricalcharacteristic of at least one of the at least two pulses of microwaveenergy is related to reflective power.
 23. A system as in claim 20,wherein the controller varies the power of each of the at least twopulses of microwave energy.
 24. A system as in claim 20, wherein thecontroller is responsive to a control input from an operator formodifying any one of the at least one pulse parameters.
 25. A method forapplying microwave energy to a target tissue, comprising the steps of:positioning a microwave energy delivery device into or adjacent aportion of the target tissue; delivering at least two pulses ofmicrowave energy to the target tissue wherein a substantial portion ofthe microwave energy is reduced between the at least two pulses ofmicrowave energy; measuring at least one of the temperature of atransmission line and a temperature of the microwave energy deliverydevice, and varying at least one of the at least two pulses of microwaveenergy in response to the at least one of the measured temperatures.